The early pull-chain flushers on toilets were placed on the left side of the tank so that people sitting could reach up and flush with their right hand.
When handles replaced the chains, the flush mechanism, although lowered, remained on the left side of the toilet.
These days, 66% of people flush when standing, using the left hand or crossing over with the right.
Even some of the sitter-downers admit to crossing over and using their left hand to flush because it wrenches the shoulder to try to flush with the right hand while sitting.
The toilet is thus one of the few devices in our modem world set up for lefties.